(1) Field of the Invention
This invention relates to a method for the production of open-cell foamed articles of cross-linked polyolefins.
(2) Description of the Prior Art
Among the methods heretofore suggested for the manufacture of cellular artlcles of cross-linked polyolefins, particularly of polyethylene, the methods which use the organic decomposition type blowing agents are generally known in the art. These methods, as disclosed in Japanese Patent Publication Nos. 8840/1965, 18832/1967 and 22674/1968, generally comprise the steps of first cross-linking polyethylene by means of an organic peroxide or by exposure to electron beams and thereafter decomposing the blowing agent thereby imparting a cellular structure to the cross-linked polyethylene. In addition, there have been known a method in which a foamable and cross-linkable composition containing a polyolefin, a blowing agent and a cross-linking agent is heated in a closed mold under increased pressure and thereafter the pressure applied to the composition in the mold is released resulting in the foamed cross-linked polyethylene, and also so-called "two-step" method, as disclosed in Japanese Patent Publication No. 29381/1970, in which the foamable and cross-linkable polyolefin composition is heated in the same manner as above to partially decompose the blowing agent and thereafter further heated under atmospheric pressure to decompose the remaining blowing agent. In the latter two methods, since the decomposition of blowing agent and cross-linking agent is effected by heating the composition in the closed mold under pressure, the cross-linking reaction of polyethylene takes place but the foaming is suppressed, and the expansion of polyethylene occurs only after the release of applied pressure. Therefore, the latter methods are same as the former methods in principle that polyethylene is first cross-linked and thereafter expanded.
The foamed products of cross-linked polyolefins obtained by the above mentioned methods have a closed cell structure. By these methods, it will be difficult to obtain a foamed product having an open cell structure. This is because, unlike the reactive foaming such as is involved in the production of polyurethane foam, the foaming of cross-linked polyolefin according to the prior methods produces closed cells, and the membranes enclosing these cells are so tough that, even under application of compressive force, they will not be ruptured to transform such closed cells into open cells and, even if the membranes are forcibly ruptured somehow or other, the ruptured cell membranes will not be retained as they are. Owing to the melt elasticity possessed commonly by polyolefins, such ruptured cell membranes cannot be retained as they are, without reference to the particular type of blowing agent and the presence or absence of cross-linking reaction. With the escape of the expanding gas, there ensues the phenomenon of contraction of cell membranes or occurrence of empty cavities. This phenomenon becomes more conspicuous with the increasing expansion degree of polyolefin foam.
In the above circumstances, the greater part of the commercially available open-cell foamed articles is polyurethane foam. However, polyolefins exhibit excellent weathering resistance as compared with the soft urethane resin typifying those resins which are capable of producing open-cell foamed articles and also have very good resistance to chemicals and to water. Thus, debut of an open-cell foamed article made from this resin has long been awaited.
So far, a few methods aimed at the production of open-cell foamed articles of polyolefins have been proposed, for instance the methods which comprises blending polyolefin with a water-soluble powder such as starch and thereafter dissolving out the water-soluble powder from the mixture, and the sintering method in which the polyolefin powder is sintered. By these methods, however, there are barely obtained cellular products of very low expansion ratio of the order of about 2 to 3 times the original volume.
Recently, there have been proposed methods which effect rupture of the membranes of closed cells of a foamed cross-linked polyethylene by the action of compressive force. One of these methods is disclosed in Japanese Patent Publication No. 10350/1974. This method comprises cooling the foamed article of a thermoplastic resin having closed cells to a temperature below the second-order transition temperature (brittle temperature) of the thermoplastic resin and roll pressing the cooled foamed article thereby producing a cellular article having open cells. This method accomplishes the transformation of closed cells to open cells by sacrificing the strength of the thermoplastic resin itself to some extent. Another of the methods is disclosed in Japanese Patent Application laid open to public inspection No. 63172/1979. This method comprises producing a foamed article of polyethylene containing an inorganic filler and subjecting the formed article to compressive force thereby rupturing the membranes of closed cells and transforming the cells into open cells. This method attains the transformation of closed cells into open cells by adding to the resin a large amount of the inorganic filler enough to lower the strength of the resin.
The former method, however, has a disadvantage that a very long time is required to cool the foamed article having extremely low thermal conductivity to a temperature below the brittle temperature (-100.degree. C.) of the resin and the method, when desired to be carried through in a short period of time, is applicable only to foamed sheets of very small thickness.
The latter method also has a disadvantage that the method itself is hardly practicable and, if it is materialized by special technical efforts, the addition of the large amount of inorganic filler inevitably decreases the degree of expansion and increases the bulk density.
In any event, successful transformation of closed cells of a foamed cross-linked polyolefin to open cells on a commercial scale remains yet to be accomplished. This is because the polyethylene resin, etc. used as the raw material of the foamed cross-linked polyolefin are so tough, by nature, that the membranes of closed cells in the foamed article will not be ruptured under application of compressive force and, even if the compressive force is great enough to rupture such membranes, the compressive force is transmitted only in the surface region of the foamed article. The compressive force transmitted to the deep portion of the foamed article is no longer great enough to rupture the membranes in that portion. Thus, the desired transformation of closed cells into open cells has not been accomplished to date.